Article Synopsis

The mechanism of formation of particulate matter (PM) in the diesel engine combustion process is outlined, and the increasingly stringent PM emissions limits in current and projected environmental legislation are noted in the context of the increasing use of fuel-efficient high-performance diesel engines in passenger cars. The types of filter systems for abating diesel particulates are described, as are the principles of filter regeneration – the controlled oxidation of PM retained in the filter, to prevent an accumulation which would ultimately block the filter and degrade engine performance. PM is characterised in terms of both particle size (coarse, accumulation mode, and nucleation mode nanoparticles) and chemical composition, and the filtration issues specific to the various PM types are outlined. Likely future trends in filter design are projected, including multifunctional systems combining PM filtration with NOx control catalysts to meet yet more stringent legislative requirements, including European Stage 5 and 6, and the so called ‘Bin 5’ levels in the U.S.A.

In the ‘bad old days’, when diesel lorries produced clouds of black smoke as they accelerated or climbed hills, a diesel engine in a car was a rarity, but during the last few years Western Europe has seen a huge increase in the production of diesel passenger cars. Today more than 50% of all new European cars have a diesel engine (1). This increased demand results from the introduction of the powerful turbocharged high-speed diesel engine that provides excellent driving characteristics with high torque at low speed, and very good fuel economy. Modern passenger car diesel engines produce much less soot or PM than did their older counterparts, because of improved fuelling and enhanced combustion characteristics. For instance, fuel pumps operating at very high pressure enable injection via several very fine nozzles into the cylinder and these injection systems permit multiple injections of fuel. In spite of the improvements in PM emissions from diesel powered vehicles, there are still concerns about the environmental consequences of these emissions. Legislation is being introduced that will demand fitment of PM filters to all diesel car models sold in Western Europe, with the implementation of the European Stage 5 emissions requirements (2) starting in 2009. In fact a growing number of new diesel passenger cars have PM filters, even though they may not be necessary to meet current legislative requirements.

The Origin of Particulate Matter

The operation of a diesel engine involves compressing air in the cylinder producing heat via the Joule-Thomson effect, and then injecting finely ‘atomised’ fuel under very high pressure (up to 2000 bar) directly into the hot gas that causes it to explosively combust. The exact details of the combustion process are the subject of active research, and it is clear that the atomised fuel droplets evaporate and burn in a fuel-rich region limited by ingress of oxygen into the burning flame front. In the fuel-rich zone, carbon forms from reactive intermediates. Subsequently when excess oxygen is present the carbon that has formed may be burnt, and if this is not completed when the combusted mixture is discharged from the cylinder through the exhaust ports, a residue of fine carbon cores remains in the exhaust gas. As the gas cools during passage into the exhaust manifold, turbocharger and the associated pipework, the carbon particles agglomerate forming high surface area material onto which uncombusted and partially combusted products adsorb, as well as sulfur oxides and nitrogen oxides (NOx) formed during the high temperature combustion in the cylinder.

When inhaled the scale of some of the smallest nanosized particles enables them to pass almost unheeded into the lungs and then even into the bloodstream. It is this mobility, coupled with the composition of the cocktail of adsorbed species, which gives rise to environmental health concerns. Figure 1 illustrates schematically the nature of diesel PM, and Figure 2 shows a chromatograph trace that indicates the very large number of different species that are adsorbed on typical diesel car PM.

Fig. 1

Schematic representation of diesel particulate matter (PM) formed during combustion of atomised fuel droplets. The resulting carbon cores agglomerate and adsorb species from the gas phase

Fig. 2

A gas chromatogram showing that a large number of organic species are adsorbed on diesel PM. Each peak corresponds to a specific compound

The amount of exhaust PM that European diesel cars are permitted to emit has decreased considerably over the last couple of decades, and this is illustrated graphically in Figure 3. The European PM emissions limits have decreased by more than an order of magnitude since 1983. Although the test conditions for each of the emission levels are not exactly the same, the overall downward trend is clear. The very low passenger car PM emissions limits for the European Stage 5 legislation, due to be phased in during 2009, can only be achieved through the fitment of filters, and legislation in other parts of the world will mean that filters will also be fitted to diesel cars elsewhere in the future.

Fig. 3

The decrease in European legislated PM passenger car emission limits. Since 1983 the permitted emissions have been reduced by an order of magnitude, and future stringent legislation will demand the fitment of filters

Diesel Particulate Filter Types

Several types of ceramic and sintered metal diesel particulate filters (DPFs) have been developed. The most successful and the most commonly used commercially, are porous ceramic wall-flow filters, as shown schematically in Figure 4. Refractory materials used to make them include cordierite, silicon carbide and aluminium titanate. Alternate channels are plugged, so the exhaust gas is forced through the channel walls. The exhaust gases pass through the walls but the PM does not and it is trapped in the filter. As PM accumulates in the filter, the backpressure across it increases, and if this continues it will become excessive, and significantly degrade engine performance – ultimately the engine will stop! It is therefore essential that the backpressure across the filter is not allowed to rise above a predetermined limit, so PM must periodically be removed from the filter to prevent this from happening. The best way of removing PM from the filter is to oxidise it to carbon dioxide (CO2) and water.

Fig. 4

A schematic representation of a ceramic wall-flow filter. The arrows indicate the gas flow through the walls. PM is trapped in the upstream side of the filter, and periodically this has to be removed to prevent unacceptable pressure-drop across the filter (Courtesy of Corning Inc)

Filter Regeneration

The process of oxidising retained PM in a diesel filter is called regeneration (3, 4). The temperature of diesel passenger car exhaust gas rarely exceeds 250°C during urban driving, so the use of nitrogen dioxide (NO2) as shown in Equations (i) and (ii) for combustion of trapped PM (written as “CH”) that takes place at temperatures in the range 250 to 400°C can only remove some of the accumulated soot when suitable temperature conditions are achieved:

2NO + O2 → 2NO2 (i)

5NO2 + 2“CH” → 5NO + 2CO2 + H2O (ii)

In contrast, heavy-duty trucks and buses operate at higher temperatures and therefore the regeneration with NO2 is very effective and continuously cleans the filter. Whilst the exhaust gas temperature for cars is too low for this regeneration method when driving in urban conditions, at speeds of around 100 km h−1, the exhaust gas temperature can be sufficiently high for nitric oxide (NO) in the exhaust gas to be oxidised over a platinum catalyst, producing NO2 which can in turn oxidise retained PM in the filter, as in Equation (ii). This type of regeneration is called ‘passive regeneration’. But to provide a regeneration method for all driving conditions, an ‘active’ form of regeneration must be employed that periodically increases the exhaust gas temperature to burn PM in the filter with oxygen (typically 550 to 600°C) every 400 to 2000 km, depending on the actual driving conditions. Three commercial filter systems developed for cars using active periodic oxygen regeneration are illustrated in Figure 5. ‘Generation 1’ employs one or two platinum-based oxidation catalysts in front of the filter to control hydrocarbons (HCs) and carbon monoxide (CO) emissions. The catalyst also oxidises extra partially burnt fuel when it is injected into the engine, to raise the exhaust temperature for active PM combustion with oxygen (5). This system was introduced in 1999 (6) and uses a base metal fuel additive to lower the temperature for PM combustion with oxygen. The first fuel additive was based on ceria, and others now in use contain base metals such as iron or strontium, and one based on platinum has been described. These multicomponent systems work well, although they are costly and fuel additive residues are retained in the filter as inorganic ash (see below), and this contributes to a higher backpressure across the filter than would be the case if no fuel additive were used.

Fig. 5

Three filter systems used on diesel cars:

Generation 1: there is a platinum oxidation catalyst before the filter to periodically burn partially combusted fuel to achieve high temperatures, and a fuel additive is used to lower the PM combustion temperature;

Generation 2: no additive is employed, the filter contains catalyst to accelerate PM combustion; Generation 3: all of the required catalyst functionality is incorporated in a single filter.

‘Generation 2’ has the advantage of using no fuel additive. As well as one or two upstream oxidation catalysts, the filter has catalyst in the walls to promote PM combustion, and today many cars use this configuration. The more recently introduced (2005) ‘Generation 3’ by Johnson Matthey requires neither a fuel additive nor an upstream catalyst. It comprises a single catalysed filter, incorporating all of the oxidation catalyst functionality to oxidise HC and CO during normal driving, and to periodically oxidise extra partially burnt fuel to raise the temperature sufficiently to combust PM with oxygen during active regenerations. Under some conditions, the catalyst might also oxidise some NO to NO2 to provide some passive PM removal during high-speed driving. This system is thermally the most efficient, because during active regeneration there is only the filter to heat, which is mounted actually on the engine turbocharger so as to minimise heat losses. The oxidation reactions used to boost the temperature actually take place in the filter, in the same location as the retained PM (7, 8). In contrast, systems with a separate upstream catalyst lose some of the heat provided during regenerations to the surroundings via the pipework between the turbocharger and the filter and so are less thermally efficient.

Particulate Matter Nanoparticles

Figure 6 shows the range of particle sizes typically present in diesel exhaust gas. Filters can remove the larger, coarse, micron-sized PM and ‘accumulation mode’ particles above 100 nm in size that together account for almost all of the PM mass. Very small nanoparticles, about 10 nm and even smaller in size, are now being addressed because when they are inhaled they can pass through the bronchial tissue into the bloodstream. Although collectively they have very little mass, they can be present in huge numbers. Recent research (9) indicates that most of this ‘nucleation mode’ PM comes from volatile organic or inorganic precursors that are formed as the exhaust gas cools. Laboratory and on-road studies on heavy-duty diesel engines show that when hot, the platinum oxidation catalyst can effectively remove all the HCs in the exhaust gas. Then, most of the nucleation mode PM is inorganic ‘sulfate’, probably as sulfuric acid, ammonium sulfate ((NH4)2SO4) or ammonium hydrogen sulfate (NH4HSO4) derived from sulfur compounds originally present in the diesel fuel and lubrication oil and traces of ammonia (NH3) present in the air. The lifetime of this PM is expected to be short, because such very fine particles coalesce and undergo other processes that take them out of the air (10). As expected, they are not formed if the sulfur concentration in the fuel and oil is reduced to below a critical level. Research in this area is very active, and more work is needed to obtain a full understanding of the nature and reactivity of nanoparticles from diesel exhaust gas. However, recently it was shown that careful chemical design of catalytic filter systems can control emissions of nanoparticles, as well as the coarser types of PM (11), and work in this area is continuing.

Fig. 6

Classification of diesel engine PM according to size. Most of the PM mass (dashed line) is associated with the accumulation-mode (∼ 100 nm) and coarse-mode particles, but there are many more nanoparticles (solid line) in the nucleation mode (∼ 10 nm) that are so small they can penetrate the human bronchial system (Courtesy of Professor David B. Kittelson, Center for Diesel Research, University of Minnesota, U.S.A.)

Inorganic Ash

Inorganic compounds are added to lubrication oils as viscosity modifiers and to provide antiwear and antioxidant properties, and to keep solid matter, especially soot, in suspension. The more commonly used compounds contain elements such as phosphorus, calcium, zinc, magnesium and sulfur (12, 13). These elements can be present in the exhaust gas, having originated from the small amounts of oil burnt in the cylinder, and they are retained as stable compounds in the filter. Similarly, inorganic species derived from the fuel are also trapped in the filter. As mentioned above, PM combustion aids are used as fuel additives in the first generation of filters on passenger cars, and these can include compounds of elements such as cerium, iron or strontium.

Because of the very high temperatures during combustion in the engine, the nature of the species present in the exhaust gas is determined by their thermodynamics (14), as is the composition of the ultimate deposit in the filter. Typically, zinc phosphate and calcium sulfate, together with material resulting from engine wear, are found in filters after a car has travelled large distances. Although the rate of ash accumulation in the filter is gradual, its presence does cause the backpressure across the filter with no PM present to increase over the lifetime of the vehicle. The gradual backpressure increase caused by accumulating inorganic ash can be minimised in three main ways: using a larger filter, using lubrication oils with reduced concentrations of inorganic additives (‘ashless oil’), and using filters with asymmetric channel structures that provide a larger inlet volume compared to that in the outlet side. The last approach may result in slightly higher backpressure for an asymmetric filter than for a symmetrical one, when fresh, but this relative difference decreases as ash accumulates in the filter and the asymmetric structure then has the lower backpressure (15). The advantage of asymmetric channels in the long term is significant, and filters of this type are likely to be used increasingly in the future. During the development of modern emissions control systems, it is essential that the performance be maintained over very many miles of use. Durability is tested both by real-world driving trials and by laboratory work, as illustrated in Figure 7.

Fig. 7

Durability testing of a compact diesel particulate filter on a vehicle. The robot (upper right insert) ‘drives’ the car in simulated service, and the emissions are measured periodically over the European test cycle to confirm that the emissions control system is working correctly

Future Filter Systems

Fitment of filters to diesel engines is environmentally important, and future legislation will demand their use in Europe and elsewhere around the world to reduce PM emissions. The overall trend in diesel emissions control systems is one of increasing complexity. Initially, platinum-based oxidation catalysts were used on diesel cars to control HC and CO emissions (16). More recently, PM filters were introduced, and the types used have evolved so that now all of the catalytic oxidation and filtration functions can be incorporated in a single relatively small filter. In the future, additional control of NOx emissions from passenger car diesel engines will be done by one of two processes. In the first, NOx is converted to nitrate species within a catalyst and they are periodically reduced to nitrogen (N2) by pulses of enriched exhaust gas obtained by late injection of fuel into the engine. This approach has the advantage that the reductant for converting NOx to N2 (diesel fuel) is already available on the vehicle. The second method uses ammonia as the reductant, which is derived from an aqueous urea ((NH2)2CO) solution that is injected into the hot exhaust gas. Over a special catalyst, the ammonia selectively reduces NOx to N2 (a process known as selective catalytic reduction (SCR)). To be cost- and space-effective, some of these functions will be combined in single components. So SCR or NOx-trapping components are likely to be incorporated into future designs of filters fitted to diesel passenger cars. When CO, HC, PM and NOx emissions are controlled by a single unit, the systems will be known as ‘four-way catalysts’ (FWCs) (17).

Conclusions

Sophisticated emissions control systems are being developed for fuel-efficient (lower CO2) modern diesel engines in passenger cars. For several years platinum-based catalysts have been fitted to diesel engine exhausts to oxidise CO and HCs, and the spotlight is now on preventing PM from entering into the atmosphere. This is done with wall-flow filters, and periodically it is necessary to combust the PM retained in the filter to prevent build-up of PM. This is done by catalytically oxidising with platinum-based catalysts extra fuel that is partially burnt in the engine to achieve the temperatures needed to burn PM with oxygen. Catalytic filter systems are capable of eliminating coarse and accumulation mode PM from diesel exhaust, and the latest and most efficient of these used on cars is mounted directly on the turbocharger in the small space in the engine compartment. The small filter contains all of the catalytic functionality to oxidise CO and HCs during normal driving, as well as to oxidise additional CO and HCs to provide sufficient temperature for regenerations with oxygen. Nanoparticles from diesel engines are the subject of much research, and ways of controlling them are understood. In the future, NOx reduction systems will be needed to meet legislative requirements, and will involve NOx-trapping technology or SCR using ammonia derived from an aqueous solution of urea. Once these approaches have been fully developed, it is likely that multifunction four-way catalyst systems will be developed, analogously to the use of three-way catalyst systems on traditional gasoline passenger cars.

The Authors

Martyn Twigg is the Chief Scientist of Johnson Matthey PLC and previously Technical Director for the Environmental Catalysts and Technologies Division. Following work at the University of Toronto, Canada, and a fellowship at the University of Cambridge, U.K., he joined ICI where he aided the development and production of heterogeneous catalysts used in the production of hydrogen, ammonia and methanol. Martyn has authored or co-authored many research papers, written numerous chapters in encyclopedic works, and edited and contributed to several books. He edits a book series on fundamental and applied catalysis.

Paul Phillips is the European Diesel Development Manager for the Environmental Technologies Division of Johnson Matthey PLC, Royston. He is responsible for the development of oxidation catalysts, the latest generation of catalysed soot filters, and NOx reduction technologies for diesel powered vehicles. Paul has a B.Sc. in chemistry and a Ph.D. in organometallic chemistry of main group elements from the University of Warwick, U.K.